1. 1. Human ceruloplasmin effectively protects washed rat erythrocytes against copper ion stimulated lysis. 2. 2. Experiments suggest that the protective action is not associated with (a) the oxidase activity of ceruloplasmin, (b) the protein's Superoxide radical scavenging ability, (c) its ability to weakly bind cupric ions (nonspecific binding) or (d) ceruloplasmin bound sialic acid.